It is generally desirable in optical transmission systems for optical signals to be transmitted via optical waveguides of the system at high power levels in order to maintain sufficient signal to noise ratios over extended transmission distances such that an acceptably low level of bit error rate is present in the received optical signal. Optical waveguides such as optical fibres however comprise an optical transmission medium which exhibits nonlinear effects at high power levels, resulting in degradation of the optical signal. Nonlinear effects may similarly occur within optical terminals of the system in optical transmission media of components such as optical amplifiers. The optimum power level at which optical signals could be transmitted is typically the maximum power level at which significant degradation due to nonlinearity is avoided. Since the performance of various optical components within the system having respective optical transmission media will vary with operating conditions and age or component replacement, a safety margin is allowed in setting the maximum power level when designating the system. Consequently, it is typically the case that optical transmission systems will operate at power levels which are less than the optimum power level.
A further problem is that an individual optical component forming part of the optical transmission system may suffer a sudden or gradual loss of performance but without total failure such that unacceptable degradation of system performance characterised by the presence of the products of a nonlinear process occurs. Often it may then be difficult to locate the faulty component.
Of particular concern in considering nonlinear processes are the effects of four wave mixing, particularly in relation to WDM (wavelength division multiplexed) optical transmission systems having a number of frequency channels separated by a substantially uniform frequency channel spacing such that each frequency channel may potentially become corrupted with the product of four wave mixing associated with respective pairs of other frequency channels.
In the example shown in FIG. 2(a), four channels having frequencies .omega..sub.1, .omega..sub.2, .omega..sub.3 and .omega..sub.4 are transmitted with equal power.
Considering photon interactions between the .omega..sub.2 and .omega..sub.3 channels only, the four wave mixing process may take the form EQU .omega..sub.1 =2.omega..sub.2 -.omega..sub.3= .omega..sub.2 -.DELTA..omega.Equation 1;
or EQU .omega..sub.4 =2.omega..sub.3 -.omega..sub.2= .omega..sub.3 +.DELTA..omega.Equation 2:
where .DELTA..omega. is the frequency separation between channels.
The products of four wave mixing between photons in channels .omega..sub.2 and .omega..sub.3 therefore occur at frequencies .omega..sub.1 and .omega..sub.4 as illustrated in FIG. 2(b), resulting in a loss of power from the channels .omega..sub.2 and .omega..sub.3 and interference in the wavelength bands around .omega..sub.1 and .omega..sub.4.
For an analysis of nonlinear processes the reader is referred to "Nonlinear Fiber Optics", Second Edition, Govind P. Agrawal, 1995 and in particular to Chapter 10 which deals with four-wave mixing. Problems associated with four-wave mixing are discussed in U.S. Pat. No. 5,410,624 which proposes using dissimilar wavelength channel spacings to avoid interference. The onset of nonlinearity with increasing levels of optical power is discussed in U.S. Pat. No. 5,420,868 which proposes amplitude and phase modulation of components of an optical beam to suppress Stimulated Brillouin Scattering. The cumulative effect of all degradations in a system can be determined by eye measurements as described in U.S. Pat. No. 4,823,360. The individual sources of degradation cannot however by distinguished.
It is known from U.S. Pat. No. 5,512,029 to modulate the optical signals such that each single wavelength channel is modulated with a high speed data stream and a respective low speed, small amplitude dither signal. The dither signals are mutually orthogonal pseudo-random sequences which can be reliably identified by digital correlation techniques in a performance monitoring apparatus. By monitoring a received optical signal, the performance of components of an optical transmission system in terms of the effects of random noise processes is measured by comparing the modulation depth in the decoded dither signal with the known modulation depth of the transmitted dither signal. Defects in components of the system may thereby be detected by observing changes in signal to noise measurements obtained by monitoring the dither modulation depth.
This technique however does not provide any specific sensitivity to the products of nonlinear processes. Furthermore, none of the above references provide a satisfactory technique for monitoring and controlling nonlinear processes within an optical transmission system.